The invention relates to the sorting of waste materials and more particularly to a method and apparatus for classifying waste materials.
More particularly, but not exclusively, the invention relates to the sorting of waste materials of the kind normally encountered in skips hired f or domestic use. Such waste materials often comprise a mixture of earth from gardens, brick and concrete rubble from small building works, wood, scrap metal and general household refuse including paper, light plastics packaging and broken glass. The nature of such skip waste is usually very different from that of household garbage which can be expected to be lighter in weight and to contain more wet vegetable and the like matter.
It is among the objects of the invention to provide a classifier and a classifying method particularly suited to the sorting of skip waste for recycling.
According to the invention a method of classifying waste materials comprises arranging two sieve screens, each having an inlet end and an outlet end, one above the other to define a space therebetween, the upper screen being of more coarse mesh size than the lower, supplying waste material to be classified to the inlet end of the upper screen, vibrating the screens, and blowing air between the screens from the inlet end and towards the outlet end, whereby the waste material is separated into a plurality of fractions with fine material passing downwards through the lower screen under gravity, light material being blown away from the outlet ends of the screen, and large heavy material falling under gravity from the outlet end of the upper screen, and collecting the separated waste material fractions. The blown air is preferably of low pressure and high volume.
Preferably the method comprises inclining the upper and lower screens downwardly towards the outlet end.
The method may comprise agitating a portion of the upper screen relatively vigorously, e.g. in a manner such that the addition of the specified portion has a greater frequency and/or amplitude of vibration that that of the remainder of the upper screen. The portion may be a portion of the upper screen adjacent to the inlet.
The method may comprise blowing air between the screens and inclined towards the outlet end from one or more positions intermediate the inlet and outlet ends. The air blown between the screens from one or more intermediate positions is preferably of relatively high pressure and of high volume.
The method may comprise the further steps of re-sieving the large heavy waste fraction to divide the fraction into larger and smaller components, and separately collecting the larger and smaller heavy waste components.
The method may comprise the step of blowing air upwardly and in a direction away from the outlet ends of the upper and lower screens from a position adjacent to the said outlets to assist separation of the light material from the large heavy material fraction.
From another aspect the invention is apparatus for classifying waste materials comprising two sieve screens each having an inlet end and an outlet end arranged one above the other to define a space therebetween, the upper screen being of relatively coarse mesh compared to the lower screen, means for vibrating the screens and air blower means for blowing air into the space between the screens from the inlet end of the screens and towards the outlet ends, whereby waste material can be separated into a plurality of fractions with fine material passing downwards under gravity through the lower screen, light material being blown away from the outlet ends of the screens and large heavy material falling under gravity from the outlet end of the upper screen. The air is preferably blown substantially parallel to the screen surfaces. At least one additional air blower may be positioned between the inlet and outlet ends of the screens and inclined towards the screen outlet ends to assist in blowing xe2x80x98lightsxe2x80x99 e.g. paper, plastics, wood and the like, away from the outlet ends of the screens for collection elsewhere.
The first said blower may be of low pressure and high volume. The said additional blower(s) may be of higher pressure and high volume.
The pair of screens may be inclined downwardly towards the outlet ends.
Waste materials to be classified may be supplied to the inlet end of the upper screen by a hopper, chute or the like, or may be batch fed to the apparatus. A speed adjustable conveyor may be utilised to feed material from the chute or hopper to the screen inlet. A collection means for fine heavies, e.g. earth, may be disposed below the screens. The means may comprise a conveyor. Means, e.g. a conveyor or the like, may be positioned at the downstream end of the screens for removing coarse heavies, e.g. builders rubble.
If desired a third screen may be positioned below the outlets of said pair of screens, to divide the large heavy waste fraction into larger and smaller components. A fan may be positioned between the third screen and the said outlets and arranged to blow air upwardly and away from the outlets to assist in separating light waste material from the large heavy fraction.